This invention relates generally to catalytic bodies, and has an especially important use in electrodes for electrochemical processes, water electrolysis and fuel cells, and catalysts for chemical processes. These electrodes possess a greatly enhanced catalytically active surface with the number and type of catalytic sites desired, due to the unique electronic and compositional states and structural configurations attainable with the amorphous materials fabricated through the process aspects of this invention. The electrodes or bodies of this invention may be formed of, or coated with, the catalytically active material of the invention. These amorphous electrode materials can be provided with a high surface to volume ratio, which further enhances electro-catalytic activity.
Some of the problems of the prior art electrodes are overvoltage and stability. Overvoltage is a source of resistance to current passage at the surface of the catalytic body. Among other factors, overvoltage is also affected by the composition, the structural configuration and the nature of the surface of the catalytic body. For each application there is a characteristic overvoltage determined by a combination of the above properties plus the dischargeing ion, electrolyte, current density, etc.
The overvoltage is also related to the number and nature of the active sites on the catalytic body which in large part determine the saturation current density of the reaction. An insufficient number of the desired type of sites restricts the rate of the desired reaction and hence the formation rate of the desired products.
As one application example, in the chlor-alkali electrochemical cell process a sodium chloride solution is electrolyzed to give chlorine gas at the anode, and hydrogen gas and a solution of sodium hydroxide at the cathode. Conventional cathodes, such as steel and the like, in such cells exhibit an overvoltage for hydrogen of approximately 300-500 mv at a current density of 2 KA/M.sup.2. This overvoltage loss results in decreased efficiency in the generation of the products and high power consumption. Due to the present energy crisis, the evolved hydrogen is becoming of significant commercial importance as a fuel, while previously it was usually vented to the atmosphere.
Using conventional electrodes, about 10% of the electrical energy used in the cells is consumed by cathode overvoltage. Hence, even a small reduction in the overvoltage will result in a meaningful energy savings. Similar overvoltage losses are present in all electrochemical processes, and similar savings are possible using the electrodes of the present invention.
The second problem encountered in the prior art was that of electrode instability. Many of the materials used are degraded by the action of the environment to which they are subjected; still others are sensitive to atmospheric oxygen, and thus must be handled with great care to prevent degradation. Another instability problem is present when a reverse current pulse is applied to the electrode. The reverse current pulse causes reverse polarization of the electrode body, which in turn causes a significant decrease in the reaction efficiency. Such current reversals are not uncommon in industrial application caused by leakage currents during startups, shutdowns and power failure. Therefore, the ability to withstand such polarization reversals is quite important.
Considerable effort has been directed toward overcoming the problems of overvoltage and stability. The problems of some applications can be partially overcome by utilizing electrodes made of or coated with compositions of noble metals such as platinum, palladium, ruthenium and the like. While these materials may improve overvoltage values, they suffer from problems of very high cost and scarcity of materials and difficulty in manufacturing procedures. Also, some of these aforementioned electrode materials are quite susceptible to degradation of performance by atmospheric contamination, or "poisoning" by certain components of the reaction mixture. In spite of these problems they have found some utility since they are heretofore the only materials suitable for certain applications, for example as electrodes for high temperature fuel cells.
Prior to the present invention, attempts to eliminate the use of noble metals have not proven wholly successful. For example, electrodes made of steel and the like, have been coated by electroplating the same with various materials providing crystalline coatings thereon. While such electrodes provided somewhat reduced hydrogen overvoltages when operated in a chlor-alkali cell, they were subject to corrosion and degradation when reverse polarized. U.S. Pat. Nos. 4,033,837 and 4,105,531 disclose electroplating an alloy of nickel (80-20%), molybdenum (10-20%) and vanadium (0.2-1.5%) on a conductive electrode to provide a material for use as a chlor-alkali cathode. This material had a somewhat lower overvoltage than uncoated steel, but suffered from degradation when subjected to reverse polarization.
U.S. Pat. No. 4,080,278 discloses electrodes coated with a compound of the general formula A.sub.X B.sub.Y O.sub.z where A is an alkali or lanthanide metal, B is chosen from the group: Ti, W, Mo, Mn, Co, V, Nb, Ta; O is oxygen. The compound is mixed with a binder metal and coated on an electrode base by techniques that include plasma and flame spraying of powdered material, vacuum evaporation, sputtering, and explosive bonding. In some cases, the techniques of the aforementioned patent may result in amorphous coatings, however it is not an object of the invention to prepare amorphous coatings, and, in fact, it appears to be the intention of the inventors to return the amorphous coating to a crystalline condition, since the latter patent refers to heating the amorphous films to return them to their crystalline state. Furthermore, no desirable properties or examples of the article thus formed are ascribed to amorphicity or vacuum deposition.
Other approaches involve materials of the "Raney" metal type, wherein deposition of a multicomponent mixture, followed by the selective removal of one of the components yields a high surface area material, with improved electro-catalytic properties. One such process is disclosed in U.S. Pat. No. 4,116,804. The process disclosed involves plating and flame spraying layers of nickel and aluminum respectively on an electrode substrate, followed by a step of heating the layers to cause interdiffusion of the metals. The interdiffused aluminum is then leached to give a high surface to volume area nickel coating. While the electrodes of this invention exhibit somewhat lowered overvoltages for electrochemical reactions, the process is quite energy intensive, and the article thus produced is quite susceptible to environmental degradation and consequently must be protected from contact with air.
Still another process is disclosed in U.S. Pat. No. 3,926,844. This process involves the deposition of amorphous borides of nickel, cobalt or iron by the reduction of their salts in an aqueous bath. While the materials thus prepared are amorphous, and do exhibit some electrocatalytic activity, the method is of limited utility. The range of compositions that can be prepared by this method is quite limited because of the compositional restrictions imposed by the process conditions involved. While low overvoltage is discussed, it does not appear that the overvoltage is in the range of the low overvoltage of the present invention and the only operating examples given are for a temperature of 20.degree. C. which is well below general industry operating temperatures which are in the range of 80.degree.-90.degree. C.
While the above prior art patents discuss improvements over various electrodes including mild steel electrodes, mild steel electrodes have high overvoltage but still remain the industry standard for the chlor-alkali industry and for hydrogen evolution in general. The prior art crystalline structures have crystalline planes and microcrystalline boundaries and dislocations, each of which increase the corrosiveness of the structures since corrosive attacks on the structure are initiated in such locations. Therefore, it appears that the prior art attempts to improve electrode performance over the mild steel electrode have not been successful, since the prior art electrodes have not been accepted to any significant degree.
One object of this invention is to provide a catalytic body and method of making the same which overcome any one and preferably all of the disadvantages of the prior art. Another object of the invention is to provide an electrode material and method of making the same where the electrode material exhibits high elecrocatalytic activity as is manifested in low overvoltage values, and also has a high degree of stability under a wide range of operating conditions including polarity reversal. It is a further object of this invention that the methods of making said electrode or catalytic body be simple and inexpensive to carry out.